you never heard of delayed pregnancy? here; allow me to educate you since your school system clearly failed you.

a sperm can stay alive in the uterus for up to however long it was since they had sex and when it would be 9 months before he got back. also the egg stays in there for longer periods than a month because it can sense when sperm is near. so no menstruation. when the mother's brain understood that her husband would be back in 9 months, it sent signals down to the uterus telling it to suck in that sperm that's been sitting there for 6 months. then in exactly 9 months, a baby comes out. or in his case, double babies as it's known in the scientific community.